The present invention relates generally to gas turbine engines, and, more specifically, to repair of rotor components thereof.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel and ignited in a combustor for generating hot combustion gases from which energy is extracted in downstream turbine stages. A typical compressor has multiple stages or rows of rotor blades and corresponding stator vanes which sequentially increase the pressure of the air as it flows in the axial downstream direction.
In a common configuration, compressor blades include integral dovetails for being removably mounted in a corresponding dovetail slot in the perimeter of a rotor disk. This permits the individual manufacture of each blade, and the individual replacement thereof in the event of blade damage during operation. However, such bladed disks require an enlarged disk rim for limiting centrifugal reaction stresses therein around the axial or circumferential dovetail slots used for radially retaining a corresponding row of rotor blades.
A modern improvement over bladed disks in a gas turbine engine is a row of rotor airfoils integrally formed with the perimeter of a rotor disk in a one-piece or unitary blisk configuration. The blade dovetails are eliminated along with the corresponding dovetail slots in the perimeter of the disk, and centrifugal loads are carried from the individual airfoils into the corresponding disk with an inherently strong loadpath therebetween. Accordingly, blisks are mechanically stronger than bladed-disks and thusly may be more efficiently configured for reducing disk size and weight for providing additional advantages and performance of the engine.
However, since the blisk airfoils are integrally formed with the supporting disk, the airfoils are not individually removable or replaceable in the event of foreign object damage thereof. Relatively small compressor blisks have been used in commercial service for many years, and are sufficiently small that they may be simply replaced in whole in the event of excessive damage to one or more of the airfoils thereof.
Alternatively, where damage is relatively minor, the damage may be simply removed, by grinding for example, thusly leaving the airfoil with a less than original configuration. This damage removal method is unacceptable for major airfoil damage since aerodynamic performance will be severely degraded, and significant rotor imbalance therefrom may be difficult to correct with ordinary balancing procedures.
Furthermore, damage removal may adversely affect strength of the airfoil itself. A typical compressor airfoil is slender and has a crescent or airfoil profile extending axially between thin leading and trailing edges. The airfoil is cantilevered from its root, with a radially opposite tip spaced closely adjacent to a surrounding casing or shroud during operation. The airfoil is typically twisted from root to tip with a complex three dimensional (3D) configuration or contour for aerodynamically pressurizing airflow during operation.
The contoured airfoil is subject to aerodynamic and centrifugal loads during operation which result in a varying pattern of stress therein. The airfoil must thusly be designed for limiting the maximum airfoil stress for enjoying a suitable useful life during operation, and the airfoil material is typically a high strength material, such as titanium, for accommodating the substantial loads carried during operation.
In the original manufacture of the blisk, its material strength must not be reduced or compromised by the various machining processes used. Excessive temperature must be avoided which would degrade material properties. For example, the machining of the individual airfoils may be done using a milling machine or an electrochemical machine having numerically controlled multiple axes for precision movement. Material is removed from the original workpiece or blank with minimal heat buildup to prevent degradation of the material strength.
Accordingly, the known repair process for compressor blisks is limited to the mere removal of airfoil damage to prevent strength reduction of the airfoil.
The advantages of using compressor blisks in a gas turbine engine are presently promoting the development of substantially larger and more expensive blisks for use in multi-stage axial compressors and low pressure fan compressors upstream therefrom. Fan blisks have relatively thick airfoils and are subject to considerably less foreign object damage than the relatively thin airfoils of compressor blisks downstream therefrom. The compressor blisks are nevertheless relatively large and quite expensive.
For example, a two stage tandem blisk includes two rows of airfoils extending from corresponding disks in a unitary assembly. Damage to any one of the blisk airfoils in either stage affects the usefulness of the entire two stage blisk. The inability to repair a two stage blisk requires the entire replacement thereof at a corresponding substantial cost.
Accordingly, it is desired to provide a method of repairing a blisk for restoring airfoils thereof to an original configuration at the repair site.
Damage in a blisk airfoil is machined away to create a notch. A repair is welded in the airfoil to fill the notch. The weld repair is then machined to restore the airfoil to a substantially original, pre-damaged configuration at the repair.